Method for producing micro-porous film of thermoplastic resin

ABSTRACT

A method for producing a microporous thermoplastic resin membrane comprising the steps of extruding a solution obtained by melt-blending a thermoplastic resin and a membrane-forming solvent through a die, cooling an extrudate to form a gel-like molding, removing the membrane-forming solvent from the gel-like molding by a washing solvent, and removing the washing solvent, the washing solvent having (a) a surface tension of 24 mN/m or less at a temperature of 25° C., (b) a boiling point of 100° C. or lower at the atmospheric pressure, and (c) a solubility of 600 ppm (on a mass basis) or less in water at a temperature of 16° C.; and the washing solvent remaining in the washed molding being removed by using warm water.

FIELD OF THE INVENTION

The present invention relates to a method for producing a microporousthermoplastic resin membrane, particularly to a method for producing amicroporous thermoplastic resin membrane while suppressing theevaporation of a washing solvent used for removing a membrane-formingsolvent and the shrinkage of the membrane.

BACKGROUND OF THE INVENTION

Microporous thermoplastic resin membranes are widely used for variousapplications such as battery separators, electrolytic capacitormembranes, various filters, moisture-permeable, waterproof clothes,reverse osmosis filtration membranes, ultrafiltration membranes,microfiltration membranes, etc.

Solvents or plasticizers are used in the production of microporousthermoplastic resin membranes by wet methods, and they should be removedfrom membranes formed from gel-like moldings to prevent them fromremaining in final products. To remove solvents or plasticizers addedfor forming membranes (membrane-forming solvents), gel-like moldings areusually washed with volatile solvents such as methylene chloride, anddried by a hot wind. However, because the microporous membranes shrinkby interface tension between the washing solvents evaporated duringdrying by a hot wind and micro-pore walls, the microporous membraneshave reduced porosity and permeability.

Thus, drying has conventionally been conducted by blowing a hot wind tothe gel-like moldings held by tenters, or bringing them to contact withmulti-stage heating rolls. Particularly when a washing solvent such ashigh-volatility methylene chloride is removed in the tentering method,however, it is impossible to grip the microporous membranes withoutdamage because of too large shrinkage of the microporous membranes. Inaddition, a large amount of a hot wind is needed in this method. In themulti-stage heating-roll method, on the other hand, small-diameter rollsare usually used, causing the problem that the microporous membranesshrink in a width direction in roll gaps. There is a drying methodcomprising a combination of the multi-stage heating roll method and thehot wind-blowing method, it is difficult to recover the washing solventby simple cooling condensation because of vigorous evaporation of thewashing solvent. In addition, when washing solvents likely to causeenvironmental contamination, Such as methylene chloride, are used in anymethods, their leakage should be prevented.

As a method for producing a microporous membrane while suppressingshrinkage during drying, JP2002-256099A proposes a method using awashing solvent having a surface tension of 24 mN/m or less at atemperature of 25° C. The use of such washing solvent suppresses theshrinkage of a network structure, which is caused by tension in agas-liquid interface in micropores during drying. However, because themicroporous membranes are dried by heating or wind, there is a problemof a slow speed of removing the washing solvents. There is further aproblem that the evaporated washing solvents tend to leak outside aproduction line.

JP2003-82151A proposes a method for removing a washing solvent whileconveying a gel-like molding by a suction roll in the production of amicroporous membrane. This method can prevent the washing solvent fromescaping by evaporation. Particularly when the gel-like molding issucked in a poor solvent to the washing solvent, a high removing effectof the washing solvent can be achieved. However, this method isdisadvantageous in providing the microporous membrane with spots bysuction grooves or apertures of a roll (suction spots).

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodfor quickly producing a microporous thermoplastic resin membrane havingexcellent appearance, while suppressing the evaporation of a washingsolvent used for removing a membrane-forming solvent and the shrinkageof the membrane.

DISCLOSURE OF THE INVENTION

As a result of intense research in view of the above object, theinventors have found that (a) by using a low-surface-tension,low-water-solubility washing solvent to remove a membrane-formingsolvent, and by using warm water to remove the washing solvent remainingin the washed molding, it is possible to quickly produce a microporousthermoplastic resin membrane having excellent appearance whilesuppressing the evaporation of the washing solvent and the shrinkage ofthe membrane, and that (b) when the poor solvent is caused to passthrough the washed molding by the suction means to remove the washingsolvent, in a state that the washed molding is in contact with a poorsolvent to the washing solvent, after the membrane-forming solvent isremoved by the washing solvent, it is possible to quickly produce amicroporous thermoplastic resin membrane having excellent appearancewhile suppressing the evaporation of the washing solvent and theshrinkage of the membrane by adjusting contact time between the washedmolding and a suction means. The present invention has been completedbased on such findings.

Thus, the first method of the present invention for producing amicroporous thermoplastic resin membrane comprises the steps ofextruding a solution obtained by melt-blending a thermoplastic resin anda membrane-forming solvent through a die, cooling an extrudate to form agel-like molding, removing the membrane-forming solvent from thegel-like molding by a washing solvent, and removing the washing solvent,the washing solvent having (a) a surface tension of 24 mN/m or less at atemperature of 25° C., (b) a boiling point of 100° C. or lower at theatmospheric pressure, and (c) a solubility of 600 ppm (on a mass basis)or less in water at a temperature of 16° C.; and the washing solventremaining in the washed molding being removed by using warm water.

The lower limit temperature of warm water is preferably the boilingpoint of the washing solvent (called “washing solvent A” unlessotherwise mentioned) −5° C. or higher, more preferably the boiling pointor higher, particularly the boiling point +3° C. or higher. The upperlimit temperature of warm water is preferably equal to or lower than thecrystal dispersion temperature of the thermoplastic resin, morepreferably equal to or lower than the crystal dispersion temperature −5°C. The removal of the washing solvent A from a molding after washing(called “washed molding” unless otherwise mentioned) can be carried outby a method of showering warm water onto the washed molding, a method ofimmersing the washed molding in warm water, or their combined methods.The method of showering warm water onto the washed molding preferablycomprises showering the warm water onto a portion of the washed moldingengaging a roll while continuously conveying the washed molding by theroll. The method of immersing the washed molding in warm waterpreferably comprises vibrating the washed molding in the warm water, orimmersing at least a portion of the washed molding engaging a roll whilecontinuously conveying the washed molding by the roll. The contact timeof the washed molding with warm water is preferably 15 seconds or less.

To obtain a microporous thermoplastic resin membrane having excellentproperties, the washing solvent A preferably meets the followingconditions (1)-(11).

(1) It has a surface tension of 20 mN/m or less at a temperature of 25°C.

(2) It has a boiling point of 80° C. or lower at the atmosphericpressure.

(3) It has solubility of 300 ppm (on a mass basis) or less in water at atemperature of 16° C.

(4) It is at least one selected from the group consisting ofhydrofluorocarbons, hydrofluoroethers, perfluorocarbons,perfluoroethers, n-paraffins having 5-7 carbon atoms, isoparaffinshaving 5-7 carbon atoms, and cycloparaffins having 5-7 carbon atoms.

(5) The hydrofluorocarbon described in (4) above is a linearhydrofluorocarbon represented by the composition formula of C₅H₂F₁₀.

(6) The hydrofluoroether described in (4) above is a compoundrepresented by the composition formula of C₄F₉OCH₃ or C₄F₉OC₂H₅.

(7) The perfluorocarbon described in (4) above is a compound representedby the composition formula of C₆F₁₄ or C₇F₁₆.

(8) The perfluoroether described in (4) above is a compound representedby the composition formula of C₄F₉OCF₃ or C₄F₉OC₂F₅.

(9) The n-paraffin having 5-7 carbon atoms described in (4) above is atleast one selected from the group consisting of n-pentane, n-hexane andn-heptane.

(10) The isoparaffin having 5-7 carbon atoms described in (4) above isat least one selected from the group consisting of 2-methylpentane,3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylhexane,3-methylhexane, 3-ethylpentane, 2,2-dimethylpentane,2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, and2,2,3-trimethylbutane.

(11) The cycloparaffin having 5-7 carbon atoms described in (4) above isat least one selected from the group consisting of cyclopentane,cyclohexane and methylcyclopentane.

The removal of the membrane-forming solvent by the washing solvent canbe conducted by two or more steps. In this case, as long as the washingsolvent A is used at least in a final step, a washing solvent other thanthe washing solvent A (called “washing solvent B” unless otherwisementioned) may be used. The washing solvent A may be used alone, or bothof the washing solvent A and the washing solvent B may be used. Thewashing stage is not restricted to two steps but may be 3 or more steps.The number of steps in the washing stage may be about 7 at most.

The washing solvent B preferably meets the following conditions(12)-(25).

(12) It is at least one selected from the group consisting of methylenechloride, carbon tetrachloride, ethane trifluoride, methyl ethyl ketone,pentane, hexane, heptane, diethyl ether and dioxane.

(13) It is a nonaqueous solvent having a boiling point of 100° C. orhigher and a flashpoint of 0° C. or higher.

(14) The nonaqueous solvent described in (13) above is at least oneselected from the group consisting of n-paraffins having 8 or morecarbon atoms; n-paraffins having 5 or more carbon atoms, at least partof whose hydrogen atoms are substituted by halogen atoms; isoparaffinshaving 8 or more carbon atoms; cycloparaffins having 7 or more carbonatoms; cycloparaffins having 5 or more carbon atoms, at least part ofwhose hydrogen atoms are substituted by halogen atoms; aromatichydrocarbons having 7 or more carbon atoms, aromatic hydrocarbons having6 or more carbon atoms, at least part of whose hydrogen atoms aresubstituted by halogen atoms; alcohols having 5-10 carbon atoms, pail ofwhose hydrogen atoms may be substituted by halogen atoms; esters having5-14 carbon atoms, pail of whose hydrogen atoms may be substituted byhalogen atoms; ethers having 4-14 carbon atoms, part of whose hydrogenatoms may be substituted by halogen atoms; and ketones having 5-10carbon atoms.

(15) The n-paraffin having 8 or more carbon atoms described in (14)above more preferably has 8-12 carbon atoms, specifically at least oneselected from the group consisting of n-octane, n-nonane, n-decane,n-undecane and n-dodecane.

(16) The n-paraffin having 5 or more carbon atoms, at least part ofwhose hydrogen atoms are substituted by halogen atoms, which isdescribed in (14) above, is at least one selected from the groupconsisting of 1-chloropentane, 1-chlorohexane, 1-chloroheptane,1-chlorooctane, 1-bromopentane, 1-bromohexane, 1-bromoheptane,1-bromooctane, 1,5-dichloropentane, 1,6-dichlorohexane and1,7-dichloroheptane.

(17) The isoparaffin having 8 or more carbon atoms, which is describedin (14) above, is at least one selected from the group consisting of2,3,4-trimethylpentane, 2,2,3-trimethylpentane, 2,2,5-trimethylhexane,2,3,5-trimethylhexane, 2,3,5-trimethylheptane and 2,5,6-trimethyloctane.

(18) The cycloparaffin having 7 or more carbon atoms described in (14)above is at least one selected from the group consisting ofcycloheptane, cyclooctane, methylcyclohexane, cis- andtrails-1,2-dimethylcyclohexane, cis- and trans-1,3-dimethylcyclohexane,and cis- and trans-1,4-dimethylcyclohexane.

(19) The cycloparaffin having 5 or more carbon atoms, part of whosehydrogen atoms are substituted by halogen atoms, which is described in(14) above, is at least one selected from the group consisting ofchlorocyclopentane and chlorocyclohexane.

(20) The aromatic hydrocarbon having 7 or more carbon atoms described in(14) above is at least one selected from the group consisting oftoluene, o-xylene, m-xylene and p-xylene.

(21) The aromatic hydrocarbon having 6 or more carbon atoms, part ofwhose hydrogen atoms are substituted by halogen atoms, which isdescribed in (14) above, is at least one selected from the groupconsisting of chlorobenzene, 2-chlorotoluene, 3-chlorotoluene,4-chlorotoluene, 3-chloro-o-xylene, 4-chloro-o-xylene,2-chloro-m-xylene, 4-chloro-m-xylene, 5-chloro-m-xylene, and2-chloro-p-xylene.

(22) The alcohol having 5-10 carbon atoms, part of whose hydrogen atomsmay be substituted by halogen atoms, which is described in (14) above,is at least one selected from the group consisting of isopentyl alcohol,tert-pentyl alcohol, cyclopentanol, cyclohexanol, 3-methoxy-1-butanol,3-methoxy-3-methyl-1-butanol, propylene glycol n-butyl ether, and5-chloro-1-pentanol.

(23) The ester having 5-14 carbon atoms, part of whose hydrogen atomsmay be substituted by halogen atoms, which is described in (14) above,is at least one selected from the group consisting of diethyl carbonate,diethyl maleate, n-propyl acetate, n-butyl acetate, isopentyl acetate,3-methoxybutyl acetate, 3-methoxy-3-methylbutyl acetate, ethyln-butyrate, ethyl n-valerate, and 2-chloroethyl acetate.

(24) The ether having 4-14 carbon atoms, part of whose hydrogen atomsmay be substituted by halogen atoms, which is described in (14) above,is at least one selected from the group consisting of n-butyl ether,diisobutyl ether, and bischloroethyl ether.

(25) The ketone described in (14) above, which has 5-10 carbon atoms, isat least one selected from the group consisting of 2-pentanone,3-pentanone, 2-hexanone, 3-hexanone, cyclopentanone, and cyclohexanone.

The second method of the present invention for producing a microporousthermoplastic resin membrane comprises the steps of extruding a solutionobtained by melt-blending a thermoplastic resin and a membrane-formingsolvent through a die, cooling an extrudate to form a gel-like molding,removing the membrane-forming solvent from the gel-like molding by awashing solvent, and causing a poor solvent to the washing solvent topass through the washed molding by a suction means to remove the washingsolvent in a state that the washed molding is in contact with the poorsolvent, the contact time t (seconds) of the washed molding with thesuction means being in a range meeting the following general formula(1):t≦(100−T)³/(1,000×P ^(0.5)×logL)  (1),wherein T is the temperature (° C.) of the poor solvent, P is a suctionpressure (kPa), and L is the size of penetrating apertures of thesuction means for sucking the washing solvent [diameter (μm) of thelargest circle inscribed in the penetrating apertures].

The contact time of the washed molding with the suction means ispreferably 0.05 seconds or more, more preferably 0.2 seconds or more.The suction means is preferably at least one selected from the groupconsisting of a suction roll, a wire roll, a slit roll and a punchedroll, particularly the wire roll. The size of the penetrating aperturesis preferably 10-5,000 μm, more preferably 20-2,000 μm, particularly50-500 μm. The suction pressure is preferably 0.5-60 kPa, morepreferably 1-40 kPa, particularly 3-20 kPa. The temperature of the poorsolvent is preferably from the boiling point of the washing solvent −10°C. to the boiling point +50° C., more preferably from the boiling pointof the washing solvent to the boiling point +50° C., particularly fromthe boiling point of the washing solvent +3° C. to the boiling point+50° C. The poor solvent is preferably water.

The contact of the washed molding with the poor solvent is preferablyconducted by a method of shivering the poor solvent onto a portion ofthe washed molding engaging the roll, a method of immersing at least aportion of the washed molding engaging the roll in the pool solvent, ortheir combined methods.

The thermoplastic resin preferably meets the following conditions(26)-(35).

(26) It is at least one selected from the group consisting ofpolyolefins, polyesters, polyamides, polyarylene ethers and polyarylenesulfides.

(27) The polyolefins described in (26) above are polyethylene orpolyethylene compositions.

(28) The polyethylene described in (27) above has a mass-averagemolecular weight of 1×10⁴ to 5×10⁶.

(29) The polyethylene described in (28) above has a mass-averagemolecular weight of 1×10⁵ to 4×10⁶.

(30) The polyethylene described in any of (27)-(29) above is at leastone selected from the group consisting of ultra-high-molecular-weightpolyethylene, high-density polyethylene, intermediate-densitypolyethylene, and low-density polyethylene.

(31) The polyethylene described in any of (27)-(30) above isultra-high-molecular-weight polyethylene having a mass-average molecularweight of 5×10⁵ or more.

(32) In the polyethylene described in any of (27)-(31) above, a ratioMw/Mn (molecular weight distribution) of a mass-average molecular weight(Mw) to a number-average molecular weight (Mn) is 5-300.

(33) The polyethylene composition described in (27) above comprisesultra-high-molecular-weight polyethylene as an indispensable component,and further comprises at least one selected from the group consisting ofhigh-density polyethylene, inter mediate-density polyethylene, andlow-density polyethylene.

(34) The polyethylene composition described in (33) above comprisesultra-high-molecular-weight polyethylene having a mass-average molecularweight of 5×10⁵ or more, and high-density polyethylene having amass-average molecular weight of 1×10⁴ or more to less than 5×10⁵.

(35) The polyethylene composition described in (33) or (34) abovecomprises as an optional component at least one polyolefin selected fromthe group consisting of polypropylene having a mass-average molecularweight of 1×10⁴ to 4×10⁶, polybutene-1 having a mass-average molecularweight of 1×10⁴ to 4×10⁶, polyethylene wax having a mass-averagemolecular weight of 1×10³ to 4×10⁴, and an ethylene α-olefin copolymerhaving a mass-average molecular weight of 1×10⁴ to 4×10⁶.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph showing the appearance of the microporousmembrane of Example 1.

FIG. 2 is a photograph showing the appearance of the microporousmembrane of Comparative Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[1] Thermoplastic Resin

Thermoplastic resins usable in the production of the microporousthermoplastic resin membrane of the present invention includepolyolefins, polyesters, polyamides, polyarylene ethers and polyarylenesulfides. Among them, polyolefins are preferable. Polyolefins may bepolyolefin alone or compositions comprising two or more polyolefins.

The polyolefins may be polymers of ethylene, propylene, butene-1,pentene-1, hexene-1,4-methylpentene-1, octene, acetate vinyl, methylmethacrylate, styrene, etc. or their copolymers. Among them,polyethylene is preferable as the polyolefin. Though not particularlyrestricted, the mass-average molecular weight of polyethylene is usually1×10⁴ to 1×10⁷, preferably 1×10⁴ to 5×10⁶, more preferably 1×10⁵ to4×10⁶.

The polyethylene includes ultra-high-molecular-weight polyethylene,high-density polyethylene, intermediate-density polyethylene, andlow-density polyethylene. These types of polyethylene may be copolymerscontaining small amounts of other α-olefins. The other α-olefins thanethylene may be propylene, butene-1, pentene-1,hexene-1,4-methylpentene-1, octene, vinyl acetate, methyl methacrylate,styrene, etc. Among them, ultra-high-molecular-weight polyethylene ispreferable. The mass-average molecular weight ofultra-high-molecular-weight polyethylene is preferably 5×10⁵ or more,more preferably 1×10⁶ to 15×10⁶, particularly 1×10⁶ to 5×10⁶.

In any case of polyethylene alone or a composition of two or more typesof polyethylene, an Mw/Mn ratio (molecular weight distribution) of itsmass-average molecular weight (Mw) to its number-average molecularweight (Mn) is preferably in a range of 5-300, more preferably in arange of 10-100, though not restricted. To adjust the molecular weightdistribution, the polyethylene may be produced by a multi-stagepolymerization, though not restrictive. Of course, single-stagepolymerized polyethylene may be used.

The polyolefin compositions comprise preferably polyethylene, morepreferably the above ultra-high-molecular-weight polyethylene, as anindispensable component. The polyolefin composition comprising the aboveultra-high-molecular-weight polyethylene as an indispensable componentcomprises preferably at least one selected from the group consisting ofhigh-density polyethylene, inter mediate-density polyethylene, andlow-density polyethylene, more preferably high-density polyethylene. Themass-average molecular weights of high-density polyethylene,intermediate-density polyethylene and low-density polyethylene arepreferably 1×10⁴ or mole to less than 5×10⁵.

The polyolefin composition comprising the aboveultra-high-molecular-weight polyethylene as an indispensable componentmay contain at least one polyolefin selected from the group consistingof polypropylene having a mass-average molecular weight of 1×10⁴ to4×10⁶, polybutene-1 having a mass-average molecular weight of 1×10⁴ to4×10⁶, polyethylene wax having a mass-average molecular weight of 1×10³to 4×10⁴, and an ethylene α-olefin copolymer having a mass-averagemolecular weight of 1×10⁴ to 4×10⁶ as an optional component. The amountof these optional polyolefins is preferably 80 parts by mass or less per100 parts by mass of the entire polyolefin composition.

[2] Production Method of Microporous Thermoplastic Resin Membrane

Any of the first and second production methods of the present inventioncomprises a step (1) of melt-blending a mixture of the abovethermoplastic resin and a membrane-forming solvent to prepare athermoplastic resin solution, a step (2) of extruding the thermoplasticresin solution through a die lip and cooling the resultant extrudate toform a gel-like molding, a step (3) of removing the membrane-formingsolvent by a washing solvent, and a step (4) of removing the washingsolvent from the resultant membrane. A stretching step may be conductedbefore and/or after the step (3), if necessary. After the steps (1)-(4),a membrane-drying step, a cross-linking step with ionizing radiation, aheat-treating step, a hydrophilizing step, a coating step, etc. may beconducted. The first and second production methods will be explainedbelow in this order.

(A) First Production Method

(1) Step of Preparing Thermoplastic Resin Solution

After a proper membrane-forming solvent is added to a thermoplasticresin, a thermoplastic resin solution is prepared by melt-blending. Thethermoplastic resin solution may contain various additives such asantioxidants, ultraviolet absorbers, antiblocking agents, pigments,dyes, inorganic fillers, etc. in a range not deteriorating the effectsof the present invention, if necessary. Fine silicate powder may beadded as a pore-forming agent, for instance.

The membrane-forming solvent may be liquid solvents or solid solvents.The liquid solvents may be aliphatic or alicyclic hydrocarbons such asnonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin,and mineral oil distillates having boiling points comparable to those ofthe above hydrocarbons. To obtain the gel-like molding having a stablesolvent content, it is preferable to use a non-volatile liquid solventsuch as liquid paraffin. The solid solvents are preferably those havingmelting points of 80° C. or lower, such as paraffin waxes, cerylalcohol, stearyl alcohol, dicyclohexyl phthalate, etc. The liquidsolvent and the solid solvent may be used in combination.

The viscosity of the liquid solvent is preferably 30-500 cSt, morepreferably 50-200 cSt at a temperature of 25° C. When this viscosity isless than 30 cSt, the thermoplastic resin solution is extruded throughthe die lip unevenly, and its blending is difficult. On the other hand,when the viscosity is more than 500 cSt, the removal of the liquidsolvent is difficult.

Though not particularly restricted, the melt-blending is preferablyuniform blending in an extruder, This method is suitable for preparing ahigh-concentration thermoplastic resin solution. The melting temperatureis preferably in a range of the melting point of the thermoplastic resin+10° C. to +100° C. Specifically, the melting temperature is preferably140-230° C., more preferably 170-200° C. The melting point is determinedby differential scanning calorimetry (DSC) according to JIS K7121. Themembrane-forming solvent may be added before blending, or charged intothe extruder in an intermediate portion during blending, though it ispreferably added before blending to prepare the solution in advance. Inthe melt-blending, an antioxidant is preferably added to prevent theoxidation of the thermoplastic resin.

A ratio of the thermoplastic resin to the membrane-forming solvent inthe thermoplastic resin solution is such that the thermoplastic resin is1-50% by mass, preferably 20-40% by mass, per 100% by mass of theirsums. When the thermoplastic resin is less than 1% by mass, largeswelling or neck-in occurs at the die exit during the extrusion of thethermoplastic resin solution, resulting in decrease in the formabilityand self-support of the gel-like molding. On the other hand, when thethermoplastic resin is more than 50% by weight, the formability of thegel-like molding is deteriorated.

(2) Step of Forming Gel-Like Molding

The melt-blended thermoplastic resin solution is extruded through a dielip directly from the extruder or via another extruder, or via anotherextruder after once cooled and pelletized. The die lip used is usually asheet-forming die having a rectangular-cross-section orifice, though adouble-cylindrical hollow die lip having a circular orifice, aninflation die lip, etc. may also be used. In the case of thesheet-forming die, the gap of its die lip is usually 0.1 to 5 mm, and itis heated at 140-250° C. during extrusion. The extrusion speed of theheated solution is preferably 0.2 to 15 m/minute.

The solution thus extruded through the die lip is formed into thegel-like molding by cooling. Cooling is preferably conducted at least toa gelation temperature or lower at a speed of 50° C./minute or more.Such cooling can set a separated phase structure, in which thethermoplastic resin phase is separated to micro-phases by themembrane-forming solvent. The cooling is conducted preferably to atemperature of 25° C. or lower. Generally, the slower cooling speedprovides the gel-like molding with larger pseudo-cell units, resultingin a coarser higher-order structure. On the other hand, the highercooling speed results in denser cell units. The cooling speed less than50° C./minute leads to increased crystallinity, making it unlikely toprovide the gel-like molding with suitable stretchability. Usable as thecooling method are a method of bringing the gel-like molding intocontact with a cooling medium such as cooling air, cooling water, etc.,a method of bringing the gel-like molding into contact with a coolingroll, etc.

(3) Step of Removing Membrane-Forming Solvent

The membrane-forming solvent is removed from the above gel-like molding.The removal of the membrane-forming solvent uses a washing solvent(called “washing solvent A” unless otherwise mentioned), which has (a) asurface tension of 24 mN/m or less at a temperature of 25° C. (b) aboiling point of 100° C. or lower at the atmospheric pressure, and (c) asolubility of 600 ppm (on a mass basis) or less in water at atemperature of 16° C., and which is not compatible with thethermoplastic resin.

With a surface tension of 24 mN/m or less at a temperature of 25° C.,the washing solvent has a small interface tension with micropore walls,thereby suppressing the shrinkage and densification of the networkstructure in the subsequent step of removing the washing solvent withwait water. Accordingly, the microporous membrane is provided withimproved porosity and permeability. The term “surface tension” usedherein means a tension in an interface between a gas and a liquid, whichis measured according to JIS K 3362. The surface tension of the washingsolvent A at a temperature of 25° C. is preferably 20 mN/m or less.Though the surface tension of the washing solvent A decreases as thetemperature is elevated, a temperature range in which the washingsolvent A is used is usually equal to or lower than its boiling point.

With a boiling point of 100° C. or lower at the atmospheric pressure,the washing solvent can be removed quickly with warm water. When thisboiling point is higher than 100° C., the removal of the washing solventtakes a long period of time, resulting in low production efficiency. Ifthe removal of the washing solvent took a long period of time, themembrane would be provided with insufficient porosity and permeabilitybecause it is heated by warm water for a long period of time, eventhough the washing solvent has a surface tension of 24 mN/m or less at atemperature of 25° C. The boiling point of the washing solvent A at theatmospheric pressure is preferably 80° C. or lower.

When the washing solvent has a solubility of 600 ppm (on a mass basis)or less in water at a temperature of 16° C., it is possible to preventwater spots (blister-like spots) from being formed on the microporousmembrane during the removal of the washing solvent with warm water. Thissolubility is preferably 300 ppm (on a mass basis) or less. Thesolubility of the washing solvent A in water becomes higher as thetemperature is elevated. However, as long as the solubility is 600 ppm(on a mass basis) or less at 16° C., the microporous membrane is freefrom water spots formed during the removal of the washing solvent withwarm water.

Specific examples of the washing solvent A include, for instance,fluorides such as hydrofluorocarbons, hydrofluoroethers,perfluorocarbons, perfluoroethers, etc., n-paraffins having 5-7 carbonatoms, isoparaffins having 5-7 carbon atoms, cycloparaffins having 5-7carbon atoms, etc.

The preferred fluoride is, for instance, at least one selected from thegroup consisting of a linear hydrofluorocarbon represented by thecomposition formula of C₅H₂F₁₀, a hydrofluoroether represented by thecomposition formula of C₄F₉OCH₃ or C₄F₉OC₂H₅, a perfluorocarbonrepresented by the composition formula of C₆F₁₄ or C₇F₁₆, and aperfluoroether represented by the composition formula of C₄F₉OCF₃ orC₄F₉OC₂F₅. Because these fluorides do not destroy ozone, they havelittle influence on environment even if they are freed outside aproduction line. Also, these fluorides are less likely to be exploded byignition, because their flashpoints are 40° C. or higher (some of themhave no flashpoints).

n-Paraffins having 5-7 carbon atoms include n-pentane, n-hexane andn-heptane, preferably n-pentane. Isoparaffins having 5-7 carbon atomsinclude 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane,2,3-dimethylbutane, 2-methylhexane, 3-methylhexane, 3-ethylpentane,2,2-dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane,3,3-dimethylpentane, 2,2,3-trimethylbutane, etc. Cycloparaffins having5-7 carbon atoms include cyclopentane, cyclohexane andmethylcyclopentane, preferably cyclopentane having a surface tension of24 mN/m or less at a temperature of 20° C.

Among those listed as examples of the washing solvent A, typicalcompounds are shown in Table 1 with respect to a surface tension, aboiling point and a solubility in water. TABLE 1 Boiling CompositionPoint (° C.) Formula or Name Surface Tension at Atmospheric Solubility(ppm of Compound (mN/m) at 25° C. Pressure by mass) in Water C₄F₉OCH₃ 1461 12 (25° C.) C₄F₉OC₂H₅ 17 76 <20 (25° C.)  C₆F₁₄ 12 56 ≦100 (25° C.) n-Pentane 16 36 225 (16° C.)  n-Hexane 18 68.7 13 (16° C.) n-Heptane 2098.4 Insoluble Cyclopentane 22 49.3 142 (16° C.)  Cyclohexane 24 80.7 52(16° C.)

The washing solvent A may be properly selected depending on the type ofthe membrane-forming solvent. The washing solvent A may be used alone orin combination. As long as the washing solvent A meets the aboverequirements (a)-(c), it may contain other solvents not meeting some ofthe above requirements (a)-(c). Such mixture may be, for instance, acombination of at least one selected from the group consisting of theabove fluorides, the above n-paraffins having 5-7 carbon atoms, theabove isoparaffins having 5-7 carbon atoms and the above cycloparaffinshaving 5-7 carbon atoms, and a small amount of a cyclichydrofluorocarbon represented by the composition formula of C₅H₃F₇, forinstance, or an aliphatic ether, an aliphatic ketone, an aliphaticalcohol, an aliphatic ester, etc. each having a boiling point of 100° C.or lower. Different washing solvents A may be used to conduct two ormore washing steps.

Before using the washing solvent A, a washing solvent (called “washingsolvent B” unless otherwise mentioned) other than the washing solvent Amay be used to remove the membrane-forming solvent. With two or morewashing steps using the washing solvents A and B properly selecteddepending on the type of the membrane-forming solvent, further improvedwashing effects can be obtained. The use of the washing solvent A in thefinal washing step can remove the washing solvent B used in the previousstep, thereby preventing the shrinkage and densification of a networkstructure, which would occur in a subsequent step of removing thewashing solvent with warm water. The treatment of the molding, fromwhich the membrane-forming solvent has been removed by the washingsolvent B, with the washing solvent A is called “rinsing treatment”hereinafter.

The washing solvent B need only have no compatibility with thethermoplastic resin, and its examples include nonaqueous solvents suchas chlorinated hydrocarbons, fluorohydrocarbons, paraffins, aromatics,alcohols, esters, ether, ketones, etc.

Preferable among the above nonaqueous solvents are, for instance,chlorinated hydrocarbons such as methylene chloride, carbontetrachloride, etc.; fluorohydrocarbons such as ethane trifluoride,etc.; ethers such as n-pentane diethyl ether, dioxane, etc.; methylethyl ketone, etc., which are generally used as solvents for removingthe membrane-forming solvent.

The other preferred examples of the above nonaqueous solvents includethose having surface tensions of 24 mN/m or less at any temperatures of80° C. or lower. The use of such nonaqueous solvent can suppress theshrinkage of the membrane, even when a relatively large amount of anonaqueous solvent is evaporated from the membrane during washing. Suchnonaqueous solvents include, for instance, n-pentane, hexane, heptane,ethane trifluoride, diethyl ether, 2-methylpentane, 3-methylpentane,cyclohexane, cyclopentane, acetone, methyl ethyl ketone, etc.

The other preferred examples of the above nonaqueous solvents includethose having boiling points of 100° C. or higher and flashpoints of 0°C. or higher at the atmospheric pressure. Such nonaqueous solvents areusable safely, because they are less volatile with little influence onthe environment and little likelihood of explosion by ignition. They arealso easily condensed because of high boiling points, resulting in easycollection and recycling. The “flashpoint” is measured herein accordingto JIS K 2265. The above flashpoint is preferably 5° C. or higher, morepreferably 40° C. or higher.

Preferable as the nonaqueous solvent having a boiling point of 100° C.or higher and a flashpoint of 0° C. or higher is at least one selectedfrom the group consisting of n-paraffins having 8 or more carbon atoms;n-paraffins having 5 or more carbon atoms, at least part of whosehydrogen atoms are substituted by halogen atoms; isoparaffins having 8or more carbon atoms; cycloparaffins having 7 or more carbon atoms;cycloparaffins having 5 or more carbon atoms, at least part of whosehydrogen atoms are substituted by halogen atoms; aromatic hydrocarbonshaving 7 or more carbon atoms, aromatic hydrocarbons having 6 or morecarbon atoms, at least part of whose hydrogen atoms are substituted byhalogen atoms, alcohols having 5-10 carbon atoms, pall of whose hydrogenatoms may be substituted by halogen atoms; esters having 5-14 carbonatoms, pail of whose hydrogen atoms may be substituted by halogen atoms;ethers having 4-14 carbon atoms, part of whose hydrogen atoms may besubstituted by halogen atoms; and ketones having 5-10 carbon atoms.

n-Paraffins having 8 or more carbon atoms are preferably n-octane,n-nonane, n-decane, n-undecane and n-dodecane, more preferably n-octane,n-nonane and n-decane.

n-Paraffins having 5 or more carbon atoms, at least part of whosehydrogen atoms are substituted by halogen atoms, are preferably1-chloropentane, 1-chlorohexane, 1-chloroheptane, 1-chlorooctane,1-bromopentane, 1-bromohexane, 1-bromoheptane, 1-bromooctane,1,5-dichloropentane, 1,6-dichlorohexane and 1,7-dichloroheptane, morepreferably 1-chloropentane, 1-chlorohexane, 1-bromopentane and1-bromohexane.

Isoparaffins having 8 or more carbon atoms are preferably2,3,4-trimethylpentane, 2,2,3-trimethylpentane, 2,2,5-trimethylhexane,2,3,5-trimethylhexane, 2,3,5-trimethylheptane and 2,5,6-trimethyloctane,more preferably 2,3,4-trimethylpentane, 2,2,3-trimethylpentane,2,2,5-trimethylhexane and 2,3,5-trimethylhexane.

Cycloparaffins having 7 or more carbon atoms are preferablycycloheptane, cyclooctane, methylcyclohexane, cis- andtrans-1,2-dimethylcyclohexane, cis- and trans-1,3-dimethylcyclohexane,and cis- and trans-1,4-dimethylcyclohexane, more preferablymethylcyclohexane.

Cycloparaffins having 5 or more carbon atoms, at least pair of whosehydrogen atoms are substituted by halogen atoms, are preferablychlorocyclopentane and chlorocyclohexane, preferably chlorocyclopentane.

Aromatic hydrocarbons having 7 or more carbon atoms are preferablytoluene, o-xylene, m-xylene and p-xylene, more preferably toluene.

Aromatic hydrocarbons having atoms 6 or more carbon atoms, at least partof whose hydrogen atoms are substituted by halogen atoms, are preferablychlorobenzene, 2-chlorotoluene, 3-chlorotoluene, 4-chlorotoluene,3-chloro-o-xylene, 4-chloro-o-xylene, 2-chloro-m-xylene,4-chloro-m-xylene, 5-chloro-m-xylene, and 2-chloro-p-xylene, morepreferably chlorobenzene, 2-chlorotoluene, 3-chlorotoluene, and4-chlorotoluene.

Alcohols having 5-10 carbon atoms, part of whose hydrogen atoms may besubstituted by halogen atoms, are preferably isopentyl alcohol,tert-pentyl alcohol, cyclopentanol, cyclohexanol, 3-methoxy-1-butanol,3-methoxy-3-methyl-1-butanol, propylene glycol n-butyl ether and5-chloro-1-pentanol, more preferably 3-methoxy-1-butanol,3-methoxy-3-methyl-1-butanol, propylene glycol n-butyl ether, and5-chloro-1-pentanol.

Esters having 5-14 carbon atoms, part of whose hydrogen atoms may besubstituted by halogen atoms, are preferably diethyl carbonate, diethylmaleate, n-propyl acetate, n-butyl acetate, isopentyl acetate,3-methoxybutyl acetate, 3-methoxy-3-methylbutyl acetate, ethyln-butyrate, ethyl n-valerate, and 2-chloroethyl acetate, more preferablyisopentyl acetate, 3-methoxybutyl acetate, 3-methoxy-3-methylbutylacetate, ethyl n-butylate and 2-chloroethyl acetate.

Ethers having 4-14 carbon atoms, part of whose hydrogen atoms may besubstituted by halogen atoms, are preferably dipropylene glycol dimethylether, n-butyl ether, diisobutyl ether, and bischloroethyl ether, morepreferably dipropylene glycol dimethyl ether and bischloroethyl ether.

Ketones having 5-10 carbon atoms are preferably 2-pentanone,3-pentanone, 2-hexanone, 3-hexanone, cyclopentanone and cyclohexanone,more preferably 2-pentanone and 3-pentanone.

The washing solvent B may be properly selected depending on the type ofthe membrane-forming solvent. The washing solvent B may be used alone orin combination. Added to the washing solvent B as an optional componentC may be at least one solvent selected from the group consisting oflinear hydrofluorocarbons represented by the composition formula ofC₅H₂F₁₀, for instance, hydrofluoroethers represented by the compositionformula of C₄F₉OCH₃ or C₄F₉OC₂H₅, for instance, cyclichydrofluorocarbons represented by the composition formula of C₅H₃F₇, forinstance, perfluorocarbons represented by the composition formula ofC₆F₁₄ or C₇F₁₆, for instance, and perfluoroethers represented by thecomposition formula of C₄F₉OCF₃ or C₄F₉OC₂F₅, for instance. The additionof the above optional component C to the washing solvent B can reduceinfluence on the environment and the likelihood of explosion byignition. The amount of the optional component C added is preferably2-98 pails by mass, more preferably 5-50 parts by mass, per 100 parts bymass of the entire mixed solvent. Particularly when the washing solventB and the optional component C are mixed at such a ratio as to have asurface tension of 24 mN/m or less at any temperatures of 80° C. orlower, the shrinkage of the membrane can be suppressed even when arelatively large amount of the washing solvent B is evaporated from themembrane during washing.

Preferred combinations of the washing solvent B used in the first stepand the washing solvent A used in the second step in the washing stageare, for instance, as follows: Washing solvent B/Washing solventA=methylene chloride/C₄F₉OCH₃, methylene chloride/C₄F₉OC₂H₅, methylenechloride/C₆F₁₄, methylene chloride/C₇F₁₆, methylene chloride/n-heptane,methylene chloride/n-hexane, ethers/hydrofluoroethers,n-paraffins/hydrofluoroethers, isoparaffins/hydrofluoroethers,cycloparaffins/hydrofluoroethers, and ketones/hydrofluoroethers.Preferable among the above combinations of the washing solvent B and thewashing solvent A are methylene chloride/C₄F₉OCH₃, methylenechloride/C₄F₉OC₂H₅, methylene chloride/C₆F₁₄, methylene chloride/C₇F₁₆,methylene chloride/h-heptane, methylene chloride/n-hexane,n-heptane/C₄F₉OCF₃, and n-heptane/C₆F₁₄. It should be noted, however,that the washing is not necessarily restricted to two steps.

Washing with the washing solvent A alone or with a combination of thewashing solvent A and the washing solvent B may be conducted by three ormore steps, if necessary. Though the number of such washing steps is notparticularly restricted, it is usually 3-7, preferably 3-4.

The washing of the gel-like molding can be conducted by a method ofshowering the washing solvent onto the gel-like molding, a method ofimmersing the gel-like molding in the washing solvent, or their combinedmethods, etc. These washing treatments are preferably conducted whileconveying the gel-like molding continuously or intermittently. A meansof conveying the gel-like molding is usually a roll. When the gel-likemolding is immersed in the washing solvent while being conveyedcontinuously, the gel-like molding is caused to pass through the washingsolvent bath. When the gel-like molding is immersed in the washingsolvent while being conveyed intermittently, a portion to be washed ofthe once stopped gel-like molding is preferably immersed and vibrated ina solvent bath at about 100 rpm. The gel-like molding is preferablyvibrated while fixing a periphery of its portion to be washed to a frameplate, etc.

The amounts of the washing solvent A and the washing solvent B used arepreferably 300-30,000 parts by mass each, per 100 pails by mass of thegel-like molding. When the gel-like molding is treated with the washingsolvent A and the washing solvent B by two or more steps, the amount ofthe washing solvent A is preferably 50-200 parts by mass per 100 pailsby mass of the washing solvent B. Washing is conducted preferably untilthe amount of the membrane-forming solvent remaining in the gel-likemolding becomes less than 1% by mass of that added.

The temperature of the washing solvent A used depends on its surfacetension. Specifically, the washing solvent A is used preferably at atemperature, at which its surface tension is 24 mN/m or less, or higher.Because the surface tension of the washing solvent A is 24 mN/m or lessat highest at a temperature of 25° C., it may generally be used at roomtemperature. The washing solvent A may be heated, if necessary.

Though depending on its boiling point, the temperature of the washingsolvent B used is generally in a range of 20-80° C. When the washingsolvent B has a boiling point of 150° C. or lower, washing can beconducted at room temperature. The washing solvent B may be heated, ifnecessary. When the washing solvent B has a boiling point of higher than150° C., the washing solvent B is preferably heated because it does noteasily penetrate into the membrane at room temperature.

(4) Step of Removing Washing Solvent

The washing solvent A remaining in the molding after washing (called“washed molding” unless otherwise mentioned) is removed with warm water.With warm water used as a medium for removing the washing solvent A, thewashing solvent A quickly oozes out of the washed molding and/orevaporated. Accordingly, the washing solvent A can be removed muchfaster than when warm wind is used, resulting in increased productionefficiency. Because the washing solvent A extracted from the washedmolding is diffused mainly in warm water, the evaporation of the washingsolvent A can be suppressed by exchanging the stained warm water to afresh one at a proper frequency.

The lower limit temperature of warm water is preferably equal to orhigher than the boiling point of the washing solvent A to be removed −5°C., more preferably equal to or higher than the boiling point,particularly equal to or higher than the boiling point +3° C. When thelower limit temperature of warm water is equal to or higher than theboiling point of the washing solvent A to be removed −5° C., the washingsolvent A can be removed quickly. The upper limit temperature of warmwater is preferably equal to or lower than the crystal dispersiontemperature of the thermoplastic resin, more preferably equal to orlower than the crystal dispersion temperature −5° C. When the upperlimit temperature of warm water is higher than the crystal dispersiontemperature, the resin may be softened. As described above,polyethylene, for instance, generally has a crystal dispersiontemperature of 90° C. Even when the crystal dispersion temperature ofthe thermoplastic resin is 95° C. or higher, the temperature of warmwater is preferably 95° C. or lower, more preferably 85° C. or lower, tosuppress steam from generating from the warm water.

The contact time of the washed molding with warm water is preferably 15seconds or less. Because the washing solvent A has a boiling point of100° C. or lower, the contact time of 15 seconds or less is usuallysufficient.

The removal of the washing solvent A from the washed molding can beconducted by a method of showering warm water onto the washed molding, amethod of immersing the washed molding in warm water, or their combinedmethods, etc. These removing treatments are conducted preferably whileconveying the washed molding continuously or intermittently. The amountof warm water when it is showered onto the washed molding is preferably50-5000 ml/m², more preferably 100-2000 ml/m². When the amount of warmwater showered is less than 50 ml/m², warm water cannot be uniformlyshowered onto the washed molding. On the other hand, when it is morethan 5000 ml/m², it is difficult to control the circulation of warmwater. When the washed molding is immersed in warm water, it ispreferable to spray warm water to the washed molding in a warm waterbath, thereby accelerating the extraction of the washing solvent A, andmaking it easy for the extracted washing solvent A to be diffused inwarm water. A means for spraying warm water may be, for instance, anozzle.

When the washed molding is treated with warm water while conveyingcontinuously, a roll is preferably used, so that a portion of the washedmolding engaging the roll is showered, or that at least a portion of thewashed molding engaging the roll is immersed in warm water. In suchtreatment method, because the roll heated by warm water can quickly heatthe washed molding, the washing solvent A is removed faster. The rollmay be heated from inside, if necessary. The heating temperature of theroll is preferably equal to or lower than the crystal dispersiontemperature of the thermoplastic resin, more preferably equal to orlower than the crystal dispersion temperature −5° C.

The diameter of the roll is preferably 3-100 cm, more preferably 5-30cm. When the diameter is less than 3 cm, there is only a small contactarea between the washed molding and the roll, failing to transmit theheat of the roll to the washed molding sufficiently. On the other hand,when it is more than 100 cm, too large a facility is needed. When thewashing solvent A is removed while conveying the washed moldingcontinuously, the conveying speed of the washed molding is preferably0.5-80 m/min, more preferably 1-50 m/min, from the aspect of productionefficiency. One roll is usually enough, though pluralities of rolls maybe used, if necessary.

When the washed molding is immersed in warm water while conveyingintermittently, it is preferable to vibrate a portion of the washedmolding, from which the solvent is removed, in warm water. With aportion of the washed molding, from which the solvent is removed, fixedto a frame plate, etc., the washed molding is preferably vibrated atabout 100 rpm, for instance.

The amount of the washing solvent remaining in the microporousthermoplastic resin membrane is reduced with warm water preferably to 5%by mass or less, more preferably 3% by mass or less, per 100% by mass ofthe dried membrane. When removal is so insufficient that a large amountof the washing solvent remains in the membrane, the porosity of themembrane is lowered by a subsequent heat treatment, resulting indeteriorated permeability.

(5) Drying Step

The membrane having the washing solvent removed is dried by awind-drying method, a heat-drying method, etc. Because water has lowaffinity for the microporous membrane, water can be removed easily andquickly by blowing warm wind to the warm-water-treated membrane. Thedrying temperature is preferably equal to or lower than the crystaldispersion temperature of polyolefin, particularly 5° C. or more lowerthan the crystal dispersion temperature.

(6) Orientation Step

Before and/or after the above step (3) of removing the membrane-formingsolvent, stretching is conducted, if necessary. The stretching ispreferably conducted before the step of removing the membrane-formingsolvent. The stretching can be conducted by a usual tentering method, ausual roll method, a usual inflation method, a usual rolling method, ortheir combined methods at predetermined magnification, after heat thegel-like molding. The stretching may be conducted monoaxially orbiaxially, though the biaxial stretching is preferable. The biaxialstretching may be simultaneous or sequential, though the simultaneousbiaxial stretching is preferable. The stretching improves the mechanicalstrength of the membrane.

Though different depending on the thickness of the gel-like molding, thestretching magnification is preferably 2 folds or more, more preferably3-30 folds in the monoaxial stretching. In the biaxial stretching, thestretching magnification is preferably 3 folds or more in any direction,namely 9 folds or more in area magnification, to increase the prickingstrength of the membrane. When the area magnification is less than 9folds, the stretching is insufficient, failing to obtain ahigh-elasticity, high-strength, microporous thermoplastic resinmembrane. On the other hand, when the area magnification is more than400 folds, there are restrictions in a stretching apparatus, astretching operation, etc.

The stretching temperature is preferably equal to or lower than themelting point of the thermoplastic resin +10° C., more preferably in arange of the crystal dispersion temperature or higher and less than thecrystals melting point. When the stretching temperature is more than themelting point +10° C., the resin is molten, failing to orient molecularchains by stretching. When the stretching temperature is lower than thecrystal dispersion temperature, the resin is so insufficiently softenedthat the membrane is easily broken by stretching, failing to achievehigh-magnification stretching. In the present invention, the stretchingtemperature is usually 100-140° C., preferably 110-120° C. The crystaldispersion temperature is determined by the measurement of thetemperature characteristics of kinetic viscoelasticity according to ASTMD 4065.

(7) Cross-Linking Step

A cross-linking treatment is conducted to the dried microporous membranepreferably by ionizing radiation. Usable ionizing radiation rays areα-rays, β-rays, γ-rays, electron beams, etc. The cross-linking treatmentby ionizing radiation can be conducted with electron beams of 0.1-100Mrad and at an acceleration voltage of 100-300 kV. The cross-linkingtreatment can improve the meltdown temperature of the membrane.

(8) Heat Treatment Step

The membrane having the washing solvent removed is preferablyheat-treated. Crystals in the microporous membrane are stabilized by aheat treatment, resulting in a uniform lamella layer. The heat treatmentmay be a thermal stretching treatment, a thermal setting treatment or athermal shrinking treatment, and properly selected depending on therequired properties of the microporous membrane. These heat treatmentsare conducted at a temperature equal to or lower than the melting pointof the polyolefin microporous membrane, preferably at a temperature of60° C. or higher and the melting point −10° C. or lower.

The thermal stretching treatment may be conducted in at least onedirection by a tentering method, a roll method or a rolling method,which is usually used, to a stretching magnification of preferably1.01-2.0 folds, more preferably 1.01-1.5 folds.

The thermal setting treatment may be conducted by a tentering method, aroll method or a rolling method. The thermal shrinking treatment may beconducted by a tentering method, a roll method or a rolling method, orby using a belt conveyor or floating. The thermal shrinking treatment isconducted in at least one direction to a shrinkage range of preferably50% or less, more preferably 30% or less.

The above thermal stretching treatment, thermal setting treatment andthermal shrinking treatment may be combined. When the thermal shrinkingtreatment is conducted after the thermal stretching treatment, alow-shrinkage, high-strength, microporous membrane can be preferablyobtained.

(9) Hydrophilizing Treatment Step

The membrane having the washing solvent removed may be subjected to ahydrophilizing treatment (treatment of imparting hydrophilic property).The hydrophilizing treatment may be a monomer-grafting treatment, asurfactant treatment, a corona-discharging treatment, etc. Themonomer-grafting treatment is preferably conducted after the ionizingradiation.

The surfactant may be any one of nonionic surfactants, cationicsurfactants, anion surfactants and amphoteric surfactants, though thenonionic surfactants are preferable. The microporous membrane getshydrophilic property by a dipping method or a doctor blade method usingthe surfactant in the form of a solution in water or a lower alcoholsuch as methanol, ethanol, isopropyl alcohol, etc.

The hydrophilized microporous membrane is dried. To provide thepolyolefin microporous membrane with improved permeability, it ispreferable to conduct heat treatment at a temperature equal to or lowerthan the melting point of the polyolefin microporous membrane whilepreventing its shrinkage during drying. Such heat treatment method withshrinkage prevented may be, for instance, a method of subjecting thehydrophilized microporous membrane to the above thermal stretchingtreatment.

(10) Coating Step

To provide the microporous membrane obtained by removing the washingsolvent with improved meltdown properties when used as batteryseparators, it is covered with a porous body made of fluororesins suchas polyvinylidene fluoride, polytetrafluoroethylene, etc. or polyimides,polyphenylene sulfide, etc. Also, the microporous membrane obtained byremoving the washing solvent may be provided with improvedhigh-temperature properties when used as battery separators, by forminga thin polypropylene membrane having a racemi-diad fraction of 0.12-0.88on a surface of the microporous membrane. The racemi-diad is a structureunit having two monomer units, which are connected in an enantiomericrelation to each other.

(B) Second Production Method

The second production method is the same as the first production method,except that (i) there is no limitation in a washing solvent used in astep of removing the membrane-forming solvent, and that (ii) in awashing-solvent-removing step, a poor solvent to the washing solvent iscaused to pass through the washed molding by a suction means to removethe washing solvent, in a state that the washed molding is in contactwith the poor solvent. Accordingly, only the step of removing themembrane-forming solvent and the step of removing the washing solventwill be explained below.

(1) Step of Removing Membrane-Forming Solvent

The washing solvent usable in the second production method is notrestrictive, unless it is compatible with the thermoplastic resin. Forinstance, the above washing solvents A and B can be used. The washingmethod using the washing solvents A and B may be the same as above.

(2) Step of Removing Washing Solvent

In a state that the washed molding is in contact with a poor solvent tothe washing solvent, the poor solvent is caused to pass through thewashed molding by a suction means to remove the washing solvent. Whilesucking the washing solvent together with the poor solvent, the contacttime t (seconds) of the washed molding with the suction means shouldmeet the following general formula (I):t≦(100−T)³/(1,100×P ^(0.5)×logL)  (1),wherein T is the temperature (° C.) of the poor solvent, P is a suctionpressure (kPa), and L is the size of penetrating apertures of thesuction means [diameter (μm) of the largest circle inscribed in thepenetrating apertures]. When the contact time t exceeds the above range,the sucking force of the suction means may deform the membrane andprovide the membrane surface with suction spots, resulting in poormembrane appearance. In some cases, the membrane may be deteriorated inproperties such as air permeability, porosity, etc. To remove thewashing solvent sufficiently, the contact time t, within a range meetingthe above general formula (1), is preferably 0.05 seconds or longer,more preferably 0.2 seconds or longer.

The suction means may be a suction roll, a suction belt, etc., and thesuction roll is preferable. Using the suction roll, the gel-like moldingcan be conveyed while sucking the washing solvent through a peripheralsurface of the suction roll. Because tension is applied to the washedmolding, the shrinkage of the membrane can be suppressed. When thewashed molding is dried by a heating roll without suction, a non-uniformtension tends to be applied to the washed molding because of theevaporation of the washing solvent. Using the suction roll, the washingsolvent evaporated by heating can be removed quickly, so that enoughtension to the washed molding can be kept even at a high conveyingspeed.

Though not particularly restricted, the suction roll may comprise, forinstance, (i) a cylinder body having an evacuatable hollow space inside,and large numbers of penetrating apertures on a peripheral surfacecommunicating with the hollow space, (ii) a pair of side plates disposedon both sides of the cylinder body, at least one of which has a holecommunicating with the hollow space, and (iii) a pair of bearings eachhaving a hole communicating with the hole of the side plate. Whilerotating the suction roll by a motor, the hollow space is evacuated bysuction by a vacuum pump via the bearing hole and a pipe, to suck aliquid or a gas on the peripheral surface. Such suction rolls aredisclosed in Japanese Patent 2630870, Japanese Patent 2899226,JP63-247251A, JP63-267648A, JP4-260561A, JP8-133536A, JP8-208100A,JP9-67053A, JP2002-160857A, JP2002-255423A, etc.

Among them, at least one selected from the group consisting of a wireroll having penetrating apertures formed by gaps between wires, a slitroll having slit-like penetrating apertures, and a punched roll havingpenetrating apertures formed by punching, and the wire roll is morepreferable.

The size of penetrating apertures herein means the diameter of thelargest circle inscribed in the penetrating apertures of the suctionmeans. When a wire roll, a slit roll or a punched roll, for instance, isused as the suction means, the size of penetrating apertures is the gapbetween wires, the width of the slit, or the diameter of the maximumcircle inscribed in the punched pores.

The size of penetrating apertures is preferably 10-5,000 μm. When thesize of penetrating apertures is less than 10 μm, not only is thesuction speed of the washing solvent low, but also the microporousmembrane is likely to be provided with pinholes by metal powder, etc. ina bath for removing the sucked washing solvent. On the other hand, whenthe size of penetrating apertures is more than 5,000 μm, suction spotsare likely to be formed on the microporous membrane. The size ofpenetrating apertures is more preferably 20-2,000 μm, particularly50-500 μm.

Though not particularly restricted, the opening ratio of the suctionroll is preferably 1-50%. When this opening ratio is less than 1%, asuction force is low. On the other hand, when the opening ratio is morethan 50%, the roll has undesirably low strength. Though not particularlyrestricted, the interval of the penetrating apertures in an axialdirection of the roll is preferably 0.5-10 mm.

The diameter of the suction roll is preferably 5-500 cm, more preferably10-200 cm. When this diameter is less than 5 cm, there is a smallcontact area between the washed molding and the roll, resulting ininsufficient suction of the washing solvent. On the other hand, whenthis diameter is more than 500 cm, too large a facility is needed.

The suction pressure (difference between the atmospheric pressure andthe pressure in the hollow space of the suction means) is preferably0.5-60 kPa, more preferably 1-40 kPa, particularly 3-20 kPa. When thesuction pressure is less than 0.5 kPa, the removal of the washingsolvent is poor, and a tension is not easily applied to the washedmolding. On the other hand, when it is more than 60 kPa, suction spotsare easily formed.

The contact of the washed molding with the poor solvent can be conductedby a method of showering the poor solvent onto a portion of the washedmolding engaging the roll, a method of immersing at least a portion ofthe washed molding engaging the roll in the poor solvent, or theircombined methods, etc. These removing treatments are preferablyconducted while continuously conveying the washed molding by the suctionroll. The conveying speed by the suction roll is preferably 0.5-80m/min, more preferably 2-60 m/min, from the aspect of productionefficiency. One suction roll is usually enough, though not restrictive.

When the poor solvent is showered onto the washed molding, the amount ofthe poor solvent is preferably 50-10,000 ml/m², more preferably100-5,000 ml/m². When the amount of the poor solvent showered is lessthan 50 ml/m², the poor solvent cannot uniformly be showered onto asurface to be washed. On the other hand, when it is more than 10,000ml/m², it is difficult to control the circulation of the poor solvent.When the washed molding is immersed in the poor solvent, the poorsolvent may be sprayed onto the washed molding in a poor solvent bath.Thus, the extraction of the washing solvent is accelerated, and theextracted washing solvent is sucked and diffused in the poor solvent,resulting in improved removal efficiency.

As long as the poor solvent has poor compatibility with the washingsolvent, it is not particularly restricted. For instance, when methylenechloride or hydrofluoroether is used as the washing solvent, water ispreferable as the poor solvent. When pentane is used as the washingsolvent, water, N,N-dimethylformamide (DMF), ethylene glycol, etc. arepreferable as the poor solvent.

The poor solvent is preferably heated to accelerate the evaporation ofthe washing solvent, thus accelerating the removal of the washingsolvent. However, from the aspect of preventing the softening of thethermoplastic resin, the upper limit temperature of the poor solvent ispreferably equal to or lower than the crystal dispersion temperature ofthe thermoplastic resin, more preferably equal to or lower than thecrystal dispersion temperature −5° C. Within a range equal to or lowerthan the crystal dispersion temperature of the thermoplastic resin, thetemperature of the poor solvent is preferably the boiling point of thewashing solvent to be removed −10° C. to the boiling point +50° C., morepreferably the boiling point of the washing solvent to the boiling point+50° C., further preferably the boiling point of the washing solvent +3°C. to the boiling point +50° C. When the temperature of the poor solventis lower than the boiling point of the washing solvent −10° C., thesolvent-removing speed is low. On the other hand, when the temperatureof the poor solvent is higher than the boiling point of the washingsolvent +50° C., the evaporation of the washing solvent is likely tooccur vigorously, resulting in deteriorated membrane appearance.

When water is used as the poor solvent, its temperature is preferably30-95° C., more preferably 35-90° C., further preferably 40-85° C. Whenthe temperature of water is lower than 30° C., the solvent-removingspeed is low. When it is higher than 95° C., steam is generated toomuch, resulting in deteriorated operation efficiency,

With a heated poor solvent showered onto a portion of the washed moldingengaging the roll, or at least part of the suction roll immersed in aheated poor solvent, the suction roll is heated by the poor solvent. Ifnecessary, the suction roll may be heated by a hot-wind heater, etc. Theheating temperature of the suction roll is equal to or lower than thecrystal dispersion temperature of the thermoplastic resin, preferablyequal to or lower than the crystal dispersion temperature −5° C. Withina range equal to or lower than the crystal dispersion temperature of thethermoplastic resin, the heating temperature of the suction roll is morepreferably the boiling point of the washing solvent to be removed −10°C. to the boiling point +50° C., further preferably the boiling point ofthe washing solvent to the boiling point +50° C., particularly theboiling point of the washing solvent +3° C. to the boiling point +50° C.

[3] Microporous Thermoplastic Resin Membrane

The microporous membrane produced by the above methods usually has aporosity of 25-80%, and a thermal shrinkage ratio of 15% or less in bothmachine direction (MD) and transverse direction (TD). Particularly themembrane produced by the first method has air permeability of 10-2,000seconds/100 cc (converted to that of a 30 μm-thick membrane), free fromwater spots on the surface. The membrane produced by the second methodhas air permeability of 10-2,000 seconds/100 cc (converted to that of a20 μm-thick membrane), free from suction spots oil the surface anddeformation.

Though properly selected depending on applications, the thickness of themicroporous thermoplastic resin membrane is preferably 5-200 μm whenused for battery separators, for instance. Because the microporousthermoplastic resin membranes obtained by the production method of thepresent invention have excellent permeability, they are suitable forbattery separators, filters, etc.

The present invention will be explained in more detail with reference toExamples below without intention of restricting the scope of the presentinvention.

EXAMPLE 1

A polyethylene composition was produced by mixing polyethylene having aMw/Mn of 16, a melting point of 135° C. and a crystal dispersiontemperature of 90° C., which comprised 25% by mass ofultra-high-molecular-weight polyethylene (UHMWPE) having a mass-averagemolecular weight of 2.0×10⁶, and 75% by mass of high-densitypolyethylene (HDPE) having a mass-average molecular weight of 3.5×10⁵,withtetrakis[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate]methaneas an antioxidant in an amount of 0.375 pails by mass per 100 parts bymass of the polyethylene composition. 25 parts by mass of the resultantpolyethylene composition was supplied to a strong-kneading, double-screwextruder (internal diameter=58 mm, L/D=42), and 75 parts by mass ofliquid paraffin was introduced into the double-screw extruder throughthe side-feeder. The resultant mixture was melt-blended at 200° C. and200 rpm in the extruder to prepare a polyethylene solution.Subsequently, this polyethylene solution was extruded through a T-dieinstalled at a tip end of the extruder such that a biaxially stretchedmembrane became as thick as about 40 μm, and drawn by a cooling rollcontrolled at 50° C., to form a gel-like sheet.

The resultant gel-like sheet was biaxially stretched to 5×5 times by acontinuously orientating machine at 116° C. to form a stretchedmembrane. Set in a frame plate of 20 cm×20 cm made of aluminum (the sameframe plate used below), the resultant membrane was immersed in awashing bath of n-pentane [surface tension: 15.5 mN/m at 25° C., boilingpoint: 36° C., solubility in water: 225 ppm (on a mass basis) at 16° C.]controlled to 23° C., and washed with vibration at 100 rpm for 30seconds. The above series of washing operations were further repeated 3times, with n-pentane exchanged to fresh one in each washing operation.The washed translucent membrane, which remained fixed to the frameplate, was immersed in a warm water bath controlled to 50° C., andtreated with the warm water while vibrating until the membrane becamewhite with n-pentane extracted. It took 5 seconds to remove the washingsolvent. Water attached to the resultant membrane was then blown off byair spray, and thermally set at 122° C. for 60 seconds to produce amicroporous polyethylene membrane.

EXAMPLE 2

A biaxially stretched membrane produced in the same manner as in Example1 was fixed to a frame plate, immersed in a first washing bath ofmethylene chloride [surface tension: 27.3 mN/m at 25° C., boiling point:40.0° C., solubility in water: 20,000 ppm (oil a mass basis) at 20° C.]controlled to 23° C., and washed while vibrating at 100 rpm for 30seconds. With methylene chloride exchanged to fresh one in each washingoperation, the above series of washing operations was further twicerepeated. The membrane, which remained fixed to the frame plate, wasthen immersed in a second washing bath (rinsing bath) of methylperfluorobutyl ether [composition formula: C₄F₉OCH₃, Novec HFE-7100,available from Sumitomo 3M, surface tension: 13.6 mN/m at 25° C.,boiling point: 61° C., solubility in water: 12 ppm (on a mass basis) at25° C., flashpoint: non] controlled to 23° C. to carry out a rinsingtreatment while vibrating at 100 rpm for 20 seconds. With methylenechloride exchanged to fresh one, the above series of rinsing operationswere repeated once. The washed translucent membrane, which remainedfixed to the frame plate, was immersed in a warm water bath controlledto 70° C., and treated with the warm water while vibrating until themembrane became white with methyl perfluorobutyl ether extracted. Ittook 10 seconds to remove the washing solvent. Water attached to themembrane was then blown off by air spray, and thermally set at 122° C.for 60 seconds to produce a microporous polyethylene membrane.

EXAMPLE 3

A microporous polyethylene membrane was produced in the same manner asin Example 2, except that it was washed with n-decane [surface tension:23.4 mN/m at 25° C., boiling point: 173° C., solubility in water: 50 ppm(on a mass basis) at 20° C.] controlled to 60° C. in the first washingbath 3 times in total, and that the temperature of the warm water wasset at 80° C. It took 2 seconds to remove the washing solvent.

EXAMPLE 4

A biaxially stretched membrane having a length of 600 m and a width of0.4 m was produced in the same manner as in Example 1. The resultantstretched membrane was washed by passing through a continuous washingapparatus at a speed of 2 m/minute. The continuous washing apparatus hadthree first washing baths containing methylene chloride controlled to23° C., and two second washing baths (rinsing baths) containing methylperfluorobutyl ether controlled to 23° C. A residence time was 30seconds in each of the three first washing baths and 20 seconds in eachof the two second washing baths. The washed membrane passed through awarm water bath controlled to 70° C. to remove the washing solvent. Witha roll having a diameter of 10 cm disposed in the warm water bath suchthat a warm water surface was 2 cm below a roll axis, the membrane wassubstantially in contact with a lower circular half of the roll, and themembrane was brought into contact with the warm water in such a mannerthat it was in contact with the roll as much as possible. The warm waterwas controlled to 70° C. and continuously renewed to avoid it from beingexcessively contaminated by the washing solvent. The residence time ofthe membrane in the warm water bath, a time period in which the membranewas in contact with the warm water, was 4 seconds. After removing thewashing solvent, water attached to the membrane was blown off by airspray, and further thermally set at 122° C. for 60 seconds to produce amicroporous polyethylene membrane.

EXAMPLE 5

A biaxially stretched membrane having a length of 600 m and a width of0.4 m was produced in the same manner as in Example 1. The resultantstretched membrane was washed by passing through a continuous washingapparatus at a speed of 2 m/minute. The continuous washing apparatus hadthree first washing baths containing methylene chloride controlled to23° C., and two second washing baths (rinsing baths) containingperfluorohexane [composition formula: C₆F₁₄, Fluorinert HC-72 availablefrom Sumitomo 3M, surface tension: 12.0 mN/m at 25° C., boiling point:56° C., solubility in water: 100 ppm or less (on a mass basis) at 25°C.] controlled to 23° C. The residence time was 30 seconds in each ofthe three first washing baths and 20 seconds in each of the two secondwashing baths. The washing solvent was removed from the washed membraneby showering warm water at 75° C. With the membrane substantially incontact with an upper circular half of a roll having a diameter of 10cm, which was disposed in the bath, war-m water was showered at 5L/minute onto the moving membrane from above the roll throughpluralities of nozzles arranged in the axial direction of the roll. Thecontact time of the membrane with warm water was 4 seconds. Waterattached to the membrane was then blown off by air spray, and furtherthermally set at 122° C. for 60 seconds to produce a microporouspolyethylene membrane.

COMPARATIVE EXAMPLE 1

A microporous polyethylene membrane was produced in the same manner asin Example 1, except that the treatment of the gel-like molding in thewashing bath was conducted with n-decane controlled to 60° C. 4 times intotal, and that the temperature of the Warm water was set at 80° C. Ittook 600-900 seconds to remove the washing solvent.

COMPARATIVE EXAMPLE 2

A microporous polyethylene membrane was produced in the same manner asin Example 1, except that the treatment of the gel-like molding in thewashing bath was conducted with methylene chloride controlled to 23° C.4 times in total, and that the temperature of the warm water was set at70° C. It took 3 seconds to remove the washing solvent.

COMPARATIVE EXAMPLE 3

A microporous polyethylene membrane was produced in the same manner asin Example 1, except that the treatment of the gel-like molding in thewashing bath was conducted with diethyl ether [surface tension: 16.4mN/m at 25° C., boiling point: 35° C., solubility in water: 65,000 ppm(on a mass basis) at 20° C.] controlled to 23° C. 4 times in total, andthat the temperature of the warm water was set at 70° C. It took 2seconds to remove the washing solvent.

COMPARATIVE EXAMPLE 4

A microporous polyethylene membrane was produced in the same manner asin Example 2 except for removing the washing solvent by blowing warmwind at 70° C. It took 40 seconds to remove the washing solvent.

COMPARATIVE EXAMPLE 5

A microporous polyethylene membrane vas produced in the same manner asin Example 4 except for changing the washing solvent in the rinsing bathto methylene chloride. It took 8 seconds to remove the washing solvent.

The photographs of the surfaces of the microporous membranes of Example1 and Comparative Example 2 are shown in FIG. 1 (Example 1) and FIG. 2(Comparative Example 2). As shown in FIGS. 1 and 2, the microporousmembrane of Example 1 had a uniform surface free from water spots, whilethe microporous membrane of Comparative Example 1 had water spots on thesurface.

The properties of the microporous thermoplastic resin membrane producedin Examples 1-5 and Comparative Examples 1-5 were measured by thefollowing methods. The results are shown in Table 2.

(1) Appearance: Observed by the naked eye.

Good: No water spot was observed.

Poor: Water spots were observed.

(2) Membrane thickness: Measured by a contact thickness meter availablefrom Mitutoyo Corporation.

(3) Air permeability: Measured according to JIS P8117 (converted to thatof a 30 μm-thick membrane).

(4) Porosity: Measured by a mass method.

(5) Thermal shrinkage ratio: Shrinkage ratios were measured 3 times inMD and TD, respectively, when the microporous membrane was exposed to105° C. for 8 hours, and their average was calculated. TABLE 2 No.Example 1 Exampl 2 Example 3 Example 4 PE Composition⁽¹⁾ UHMWPE (wt.%)⁽²⁾  25  25  25  25 HDPE (wt. %)⁽³⁾  75  75  75  75 PEConcentration⁽⁴⁾  25  25  25  25 Membrane-Forming ConditionsStretching⁽⁵⁾ 5 × 5 5 × 5 5 × 5 5 × 5 Stretching Temp. (° C.) 116 116116 116 Washing Treatment Washing Solvent n-C₅H₁₂ CH₂Cl₂ n-C₁₀H₂₂ CH₂Cl₂Boiling Point⁽⁶⁾  36  40 173  40 Surface Tension⁽⁷⁾  15.5  27.3  23.4 27.3 Solubility in Water⁽⁸⁾ 225 (16° C.) 20,000 (20° C.) 50 (20° C.)20,000 (20° C.) Method⁽⁹⁾ A A A B Temperature (° C.)  23  23  60  23Time (seconds)  30  30  30  30 Number of steps  4⁽¹⁰⁾  3⁽¹⁰⁾  3⁽¹⁰⁾ 3⁽¹¹⁾ Rinsing Treatment Washing Solvent — C₄F₉OCH₃ ⁽¹²⁾ C₄F₉OCH₃ ⁽¹²⁾C₄F₉OCH₃ ⁽¹²⁾ Boiling Point⁽⁶⁾  61  61  61 Surface Tension⁽⁷⁾  13.6 13.6  13.6 Solubility in Water⁽⁸⁾ (25° C.) 12 (25° C.) 12 (25° C.)Method⁽⁹⁾ — A A B Temperature (° C.) —  23  23  23 Time (seconds) —  20 20  20 Number of Steps —  2⁽¹⁰⁾  2⁽¹⁰⁾  2⁽¹¹⁾ Washing Solvent-RemovingTreatment Removing Medium Warm Water Warm Water Warm Water Warm WaterMethod Immersion Immersion Immersion Immersion Temperature (° C.)  50 70  80  70 Time (seconds)  5  10  2  4 Thermal Setting TreatmentTemperature (° C.) 122 122 122 122 Time (seconds)  60  60  60  60Properties of Microporous Membrane Appearance Good Good Good GoodThickness (μm)  34.2  34.6  34.2  31.0 Porosity (%)  54.3  54.7  54.8 51.5 Air Permeability⁽¹³⁾ 258 263 259 320 Thermal Shrinkage Ratio⁽¹⁴⁾MD (%)  14.2  14.8  13.9  13.6 TD (%)  12.9  13.5  12.6  12.5 No.Example 5 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 PE Composition⁽¹⁾ UHMWPE(wt. %)⁽²⁾  25  25  25  25 HDPE (wt. %)⁽³⁾  75  75  75  75 PEConcentration⁽⁴⁾  25  25  25  25 Membrane-Forming ConditionsStretching⁽⁵⁾ 5 × 5 5 × 5 5 × 5 5 × 5 Stretching Temp. (° C.) 116 116116 116 Washing Treatment Washing Solvent CH₂Cl₂ n-C₁₀H₂₂ CH₂Cl₂C₂H₅OC₂H₅ Boiling Point⁽⁶⁾  40 173  40  35 Surface Tension⁽⁷⁾  27.3 23.4  27.3  16.4 Solubility in Water⁽⁸⁾ 20,000 (20° C.) 50 (20° C.)20,000 (20° C.) 65,000 (20° C.) Method⁽⁹⁾ B A A A Temperature (° C.)  23 60  23  23 Time (seconds)  30  30  30  30 Number of steps  3⁽¹⁰⁾  4⁽¹⁰⁾ 4⁽¹⁰⁾  4⁽¹⁰⁾ Rinsing Treatment Washing Solvent C₆F₁₄ ⁽¹⁵⁾ — — — BoilingPoint⁽⁶⁾  56 Surface Tension⁽⁷⁾  12 Solubility in Water⁽⁸⁾ ≦100 (25° C.)Method⁽⁹⁾ B — — — Temperature (° C.)  23 — — — Time (seconds)  20 — — —Number of Steps  2⁽¹¹⁾ — — — Washing Solvent-Removing Treatment RemovingMedium Warm Water Warm Water Warm Water Warm Water Method ShoweringImmersion Immersion Immersion Temperature (° C.)  75  80  70  70 Time(seconds)  4 600-900  3  2 Thermal Setting Treatment Temperature (° C.)122 122 122 122 Time (seconds)  60  60  60  60 Properties of MicroporousMembrane Appearance Good Good Poor Poor Thickness (μm)  31.1  23.9  30.6 28.5 Porosity (%)  51.8  37.1  49.3  47.2 Air Permeability⁽¹³⁾ 325 912379 427 Thermal Shrinkage Ratio⁽¹⁴⁾ MD (%)  13.6  10.2  12.1  11.6 TD(%)  12.7  9.9  11.1  10.5 No. Comp. Ex. 4 Comp. Ex. 5 PE Composition⁽¹⁾UHMWPE (wt. %)⁽²⁾  25   25 HDPE (wt. %)⁽³⁾  75   75 PE Concentration⁽⁴⁾ 25   25 Membrane-Forming Conditions Stretching⁽⁵⁾ 5 × 5 5 × 5Stretching Temp. (° C.) 116   116 Washing Treatment Washing SolventCH₂Cl₂ CH₂Cl₂ Boiling Point⁽⁶⁾  40   40 Surface Tension⁽⁷⁾  27.3   27.3Solubility in Water⁽⁸⁾ 20,000 (20° C.) 20,000 (20° C.) Method⁽⁹⁾ A BTemperature (° C.)  23   23 Time (seconds)  30   30 Number of steps 3⁽¹⁰⁾    3⁽¹⁰⁾ Rinsing Treatment Washing Solvent C₄F₉OCH₃ ⁽¹²⁾ CH₂Cl₂Boiling Point⁽⁶⁾  61   40 Surface Tension⁽⁷⁾  13.6   27.3 Solubility inWater⁽⁸⁾ 12 (25° C.) 20,000 (20° C.) Method⁽⁹⁾ A B Temperature (° C.) 23   23 Time (seconds)  20   20 Number of Steps  2⁽¹⁰⁾    2⁽¹¹⁾ WashingSolvent-Removing Treatment Removing Medium Warm Wind Warm Water MethodBlowing Immersion Temperature (° C.)  70   70 Time (seconds)  40    8Thermal Setting Treatment Temperature (° C.) 122   122 Time (seconds) 60   60 Properties of Microporous Membrane Appearance Poor PoorThickness (μm)  34.3   21.9 Porosity (%)  55.1   31.5 AirPermeability⁽¹³⁾ 252 2,100 Thermal Shrinkage Ratio⁽¹⁴⁾ MD (%)  12.9  13.1 TD (%)  11.6   12.4Note:⁽¹⁾Mw/Mn = 16.⁽²⁾Ultra-high-molecular-weight polyethylene, Mw = 2.0 × 10⁶.⁽³⁾High-density polyethylene, Mw = 3.5 × 10⁵.⁽⁴⁾The Concentration (wt. %) of polyethylene in the melt blend.⁽⁵⁾Magnification (folds) of simultaneous biaxial stretching in MD andTD.⁽⁶⁾Boiling point (° C.) at the atmospheric pressure.⁽⁷⁾Surface tension (mN/m) at 25° C.⁽⁸⁾Solubility (ppm by weight) in water.⁽⁹⁾A: Vibration while the frame was fixed. B: Conveying by the roll.⁽¹⁰⁾The number of washing steps.⁽¹¹⁾The number of washing baths.⁽¹²⁾Methyl perfluorobutyl ether.⁽¹³⁾Air permeability (sec/100 cc) converted to that of a 30 μm-thickmembrane.⁽¹⁴⁾Thermal shrinkage ratio (%) in MD and TD.⁽¹⁵⁾Perfluorohexane.

As shown in Table 2, the microporous thermoplastic resin membranes ofExamples 1-5 produced by the first method of the present invention hadexcellent appearance, porosity, air permeability and thermal shrinkageresistance. On the other hand, because the rinsing treatment wasconducted with a washing solvent having a boiling point of higher than100° C. in Comparative Example 1, a long period of time was needed toremove the washing solvent, and the resultant microporous thermoplasticresin membrane had poor porosity and air permeability. The microporousthermoplastic resin membranes of Comparative Examples 2 and 5 had poorappearance, porosity and air permeability, because they were subjectedto a washing treatment and/or a rinsing treatment with the washingsolvent having a surface tension of more than 24 mN/m at 25° C. and asolubility of more than 600 ppm (on a mass basis) in water at 16° C. Themicroporous thermoplastic resin membrane of Comparative Example 3 hadpoor appearance, porosity and air permeability, because it was subjectedto a washing treatment with the washing solvent having a solubility ofmore than 600 ppm (on a mass basis) in water at 16° C. In ComparativeExample 4, a long period of time was needed to remove the washingsolvent, because the washing solvent was removed with a warm wind.

EXAMPLE 6

A polyethylene composition was produced by mixing polyethylene havingMw/Mn of 16.8, a melting point of 135° C. and a crystal dispersiontemperature of 90° C., which comprised 20% by mass ofultra-high-molecular-weight polyethylene (UHMWPE) having a mass-averagemolecular weight of 2.0×10⁶, and 80% by mass of high-densitypolyethylene (HDPE) having a mass-average molecular weight of 3.5×10⁵,withtetrakis[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate]methaneas an antioxidant in an amount of 0.375 parts by mass per 100 parts bymass of the composition. 30 parts by mass of the resultant polyethylenecomposition was charged into a strong-blending double-screw extruder(inner diameter 58 mm, L/D=42), and 70 parts by mass of liquid paraffinwas supplied to this double-screw extruder through a side feeder, tocarry out melt-blending under the conditions of 210° C. and 200 rpm toprepare a polyethylene solution in the extruder. This polyethylenesolution was then extruded through a T-die installed at a tip end of theextruder such that a biaxially stretched membrane became as thick asabout 45 μm, and drawn by a cooling roll controlled at 40° C., to form agel-like sheet. The resultant gel-like molding was biaxially stretchedto 5×5 times by a continuously orientating machine at 119° C. Thebiaxially stretched membrane was then taken tip around a paper pipe withits end portions cut to have a width of 40 cm, thereby obtaining astretched membrane having a length of 600 m.

The stretched membrane was conveyed through a continuous washingapparatus at a speed of 16 m/minute for washing. The continuous washingapparatus had three first washing baths containing methylene chloride[surface tension: 27.3 in mN/m at 25° C., boiling point: 40.0° C.,solubility in water: 20,000 ppm (on a mass basis) at 20° C.] controlledto 26° C., and two second washing baths (rinsing baths) containingmethyl perfluorobutyl ether [composition formula: C₄F₉OCH₃, NovecHFE-7100 available from Sumitomo 3M, surface tension: 13.6 mN/m at 25°C., boiling point: 61° C., solubility in water: 12 ppm (on a mass basis)at 25° C., flashpoint: non] controlled to 26° C. The liquid surfaceheight in each washing bath was controlled such that the residence timein each of the first and second washing baths was 10 seconds. With eachwashing bath provided with a liquid-supplying line at a lower positionand a line for discharging an overflowing liquid at the liquid surfaceheight, each flesh liquid was continuously supplied at 4 L/min whiledischarging the old one at the same rate.

The washing solvent was removed from the washed molding by a suctionwire roll, while keeping the molding in contact with warm water at 85°C. With a suction wire roll constituted by a wound round wire having adiameter of 0.8 mm [pore size (gaps between wires): 50 m, diameter: 10cm] disposed in a warm water bath such that a warm water surface was 4cm above the lowest position of the roll, the washed molding was broughtinto contact with a lower circular half of the roll. The contact time ofthe washed molding with the suction wire roll was 0.59 seconds, and thesuction pressure was 5 kPa. The warm water was renewed by continuoussupplying and withdrawal to avoid it from being excessively contaminatedby the washing solvent. The membrane deprived of the washing solvent wasfixed to an aluminum frame of 20 cm×20 cm, and thermally set at 124° C.for 120 seconds to produce a microporous polyethylene membrane.

EXAMPLE 7

A microporous polyethylene membrane was produced in the same manner asin Example 6, except that the conveying speed of the stretched membranewas set at 8 m/minute, that the speed of supplying a flesh liquid toeach of the first and second washing baths was set at 2 L/min, that thesuction pressure was set at 20 kPa, that the temperature of the warmwater was set at 75° C., and that the contact time of the washed moldingwith the suction wire roll was set to 1.18 seconds.

EXAMPLE 8

A microporous polyethylene membrane was produced in the same manner asin Example 6, except that the washing solvent for a rinsing treatmentwas changed to methylene chloride, and that the wire gap in the suctionwire roll was set at 100 μm.

EXAMPLE 9

A microporous polyethylene membrane was produced in the same manner asin Example 6, except that the washing solvent in the first and secondwashing baths was changed to n-pentane [surface tension: 15.5 mN/m at25° C., boiling point: 36° C., solubility in water: 225 ppm (on a massbasis) at 16° C.], that the conveying speed of the stretched membranewas set at 12 m/minute, that the speed of supplying a fresh liquid toeach of the first and second washing baths was set at 3 L/min, that thewire gap in the suction wire roll was set to 200 μm, that the suctionpressure was set at 10 kPa, that the warm water temperature was set at80° C., and that the contact time of the washed molding with the suctionwire roll was set to 0.79 seconds.

EXAMPLE 10

A microporous polyethylene membrane was produced in the same manner asin Example 9 except for using N,N-dimethylformamide controlled to 80° C.as the poor solvent in place of warm water.

EXAMPLE 11

A microporous polyethylene membrane was produced in the same manner asin Example 6, except that the solvent in the first washing bath waschanged to n-decane [surface tension: 23.4 mN/m at 25° C., boilingpoint: 173° C., solubility in water: 50 ppm (on a mass basis) at 20° C.]controlled to 55° C., that the washing solvent in the rinsing bath waschanged to perfluorohexane [composition formula: C₆F₁₄, Fluorinert HC-72available from Sumitomo 3M, surface tension 12.0 m N/m at 25° C.,boiling point: 56° C., solubility in water: 100 ppm (on a mass basis) orless at 25° C.], that the wire gap in the suction wire roll was set to200 μm, that the suction pressure was set at 20 kPa, and that the warmwater temperature was set at 70° C.

COMPARATIVE EXAMPLE 6

A microporous polyethylene membrane was produced in the same manner asin Example 6, except that the conveying speed of the stretched membranewas set at 8 m/minute, that the speed of supplying a fresh liquid toeach of the first and second washing baths was set at 2 L/min, and thatthe contact time of the washed molding with the suction wire roll wasset to 1.18 seconds.

COMPARATIVE EXAMPLE 7

A microporous polyethylene membrane was produced in the same manner asin Example 6 except for changing the suction pressure to 20 kPa

COMPARATIVE EXAMPLE 8

A microporous polyethylene membrane was produced in the same manner asin Example 8, except that the conveying speed of the stretched membranewas set at 8 m/minute, that the speed of supplying a flesh liquid toeach of the first and second washing baths was set at 2 L/min, and thatthe contact time of the washed molding with the suction wire roll waschanged to 1.18 seconds.

The properties of the microporous thermoplastic resin membranes obtainedin Examples 6-11 and Comparative Examples 6-8 were measured by thefollowing methods. The results are shown in Table 3.

(1) Appearance: Observed by the naked eye.

Good: No suction spots were observed.

Poor: Suction spots were observed.

The membrane thickness (2), the porosity (3), the air permeability (4)and the thermal shrinkage ratio (5) were measured by the same methods asin Examples 1-5. The air permeability was converted to that of a 20μm-thick membrane. TABLE 3 No. Example 6 Example 7 Example 8 Example 9PE Composition⁽¹⁾ UHMWPE (wt. %)⁽²⁾ 20 20 20 20 HDPE (wt. %)⁽³⁾ 80 80 8080 PE Concentration⁽⁴⁾ 30 30 30 30 Membrane-Forming ConditionsStretching⁽⁵⁾ 5 × 5 5 × 5 5 × 5 5 × 5 Stretching Temp. (° C.) 119 119119 119 Conveying Speed (m/min) 16 8 16 12 Washing Treatment WashingSolvent CH₂Cl₂ CH₂Cl₂ CH₂Cl₂ n-C₅H₁₂ Boiling Point⁽⁶⁾ 40 40 40 36Surface Tension⁽⁷⁾ 27.3 27.3 27.3 15.5 Solubility in Water⁽⁸⁾ 20,000(20° C.) 20,000 (20° C.) 20,000 (20° C.) 225 (16° C.) Method⁽⁹⁾ B B B BTemperature (° C.) 26 26 26 26 Time (seconds) 10 10 10 10 Number ofSteps 3 3 3 3 Fresh Liquid⁽¹⁶⁾ (L/min) 4 2 4 3 Rinsing Treatment WashingSolvent C₄F₉OCH₃ ⁽¹²⁾ C₄F₉OCH₃ ⁽¹²⁾ CH₂Cl₂ n-C₅H₁₂ Boiling Point⁽⁶⁾ 6161 40 36 Surface Tension⁽⁷⁾ 13.6 13.6 27.3 15.5 Solubility in Water⁽⁸⁾12 (25° C.) 12 (25° C.) 20,000 (20° C.) 225 (16° C.) Method⁽⁹⁾ B B B BTemperature (° C.) 26 26 26 26 Time (seconds) 10 10 10 10 Number ofSteps 2 2 2 2 Fresh Liquid⁽¹⁶⁾ (L/min) 4 2 4 3 Washing Solvent-RemovingTreatment Suction Roll Wire Roll Wire Roll Wire Roll Wire Roll Diameter(μm) 10 10 10 10 Size of Apertures (cm) 50 50 100 200 Suction Pressure(kPa) 5 20 5 10 Poor Solvent Warm Water Warm Water Warm Water Warm WaterPoor Solvent Temp. (° C.) 85 75 85 80 Contact Time (seconds) 0.59 1.180.59 0.79 (100 − T)³/(1100 × P^(0.5) × log L) 0.81 1.87 0.69 1.00Thermal Setting Treatment Temperature (° C.) 124 124 124 124 Time(seconds) 120 120 120 120 Properties of Microporous Membrane AppearanceGood Good Good Good Thickness (μm) 25.9 25.9 24.8 25.5 Porosity (%) 52.352.5 50.2 51.8 Air Permeability⁽¹⁷⁾ 189 185 237 203 Thermal ShrinkageRatio⁽¹⁴⁾ MD (%) 12.6 12.7 12.1 12.4 TD (%) 11.6 11.9 11.4 11.6 No.Example 10 Example 11 Comp. Ex. 6 Comp. Ex. 7 PE Composition⁽¹⁾ UHMWPE(wt. %)⁽²⁾ 20 20 20 20 HDPE (wt. %)⁽³⁾ 80 80 80 80 PE Concentration⁽⁴⁾30 30 30 30 Membrane-Forming Conditions Stretching⁽⁵⁾ 5 × 5 5 × 5 5 × 55 × 5 Stretching Temp. (° C.) 119 119 119 119 Conveying Speed (m/min) 1216 8 16 Washing Treatment Washing Solvent n-C₅H₁₂ n-C₁₀H₂₂ CH₂Cl₂ CH₂Cl₂Boiling Point⁽⁶⁾ 36 173 40 40 Surface Tension⁽⁷⁾ 15.5 23.4 27.3 27.3Solubility in Water⁽⁸⁾ 225 (16° C.) 50 (20° C.) 20,000 (20° C.) 20,000(20° C.) Method⁽⁹⁾ B B B B Temperature (° C.) 26 55 26 26 Time (seconds)10 10 10 10 Number of Steps 3 3 3 3 Fresh Liquid⁽¹⁶⁾ (L/min) 3 4 2 4Rinsing Treatment Washing Solvent n-C₅H₁₂ C₆F₁₄ ⁽¹⁵⁾ C₄F₉OCH₃ ⁽¹²⁾C₄F₉OCH₃ ⁽¹²⁾ Boiling Point⁽⁶⁾ 36 56 61 61 Surface Tension⁽⁷⁾ 15.5 1213.6 13.6 Solubility in Water⁽⁸⁾ 225 (16° C.) ≦100 (25° C.) 12 (25° C.)12 (25° C.) Method⁽⁹⁾ B B B B Temperature (° C.) 26 26 26 26 Time(seconds) 10 10 10 10 Number ot Steps 2 2 2 2 Fresh Liquid⁽¹⁶⁾ (L/min) 34 2 4 Washing Solvent-Removing Treatment Suction Roll Wire Roll WireRoll Wire Roll Wire Roll Diameter (μm) 10 10 10 10 Size of Apertures(cm) 200 200 50 50 Suction Pressure (kPa) 10 20 5 20 Poor SolventN,N-Dimethyl- Warm Water Warm Water Warm Water formamide Poor SolventTemp. (° C.) 80 70 85 85 Contact Time (seconds) 0.79 0.59 1.18 0.59 (100− T)³/(1100 × P^(0.5) × log L) 1.00 2.39 0.81 0.40 Thermal SettingTreatment Temperature (° C.) 124 124 124 124 Time (seconds) 120 120 120120 Properties of Microporous Membrane Appearance Good Good Poor PoorThickness (μm) 24.9 25.9 25.1 24.8 Porosity (%) 49.8 52.4 50.2 50.4 AirPermeability⁽¹⁷⁾ 243 192 240 231 Thermal Shrinkage Ratio⁽¹⁴⁾ MD (%) 11.912.5 12.2 12.2 TD (%) 10.9 11.7 11.2 11.4 No. Comp. Ex. 8 PEComposition⁽¹⁾ UHMWPE (wt. %)⁽²⁾ 20 HDPE (wt. %)⁽³⁾ 80 PEConcentration⁽⁴⁾ 30 Membrane-Forming Conditions Stretching⁽⁵⁾ 5 × 5Stretching Temp. (° C.) 119 Conveying Speed (m/min) 8 Washing TreatmentWashing Solvent CH₂Cl₂ Boiling Point⁽⁶⁾ 40 Surface Tension⁽⁷⁾ 27.3Solubility in Water⁽⁸⁾ 20,000 (20° C.) Method⁽⁹⁾ B Temperature (° C.) 26Time (seconds) 10 Number of steps 3 Fresh Liquid⁽¹⁶⁾ (L/min) 2 RinsingTreatment Washing Solvent CH₂Cl₂ Boiling Point⁽⁶⁾ 40 Surface Tension⁽⁷⁾27.3 Solubility in Water⁽⁸⁾ 20,000 (20° C.) Method⁽⁹⁾ B Temperature (°C.) 26 Time (seconds) 10 Number ot Steps 2 Fresh Liquid⁽¹⁶⁾ (L/min) 2Washing Solvent-Removing Treatment Suction Roll Wire Roll Diameter (μm)10 Size of Apertures (cm) 100 Suction Pressure (kPa) 5 Poor Solvent WarmWater Poor Solvent Temp. (° C.) 85 Contact Time (seconds) 1.18 (100 −T)³/(1100 × P^(0.5) × log L) 0.69 Thermal Setting Treatment Temperature(° C.) 124 Time (seconds) 120 Properties of Microporous MembraneAppearance Poor Thickness (μm) 23.6 Porosity (%) 48 Air Permeability⁽¹⁷⁾288 Thermal Shrinkage Ratio⁽¹⁴⁾ MD (%) 11.3 TD (%) 10.4Note:⁽¹⁾⁻⁽¹⁵⁾The same as in Table 2.⁽¹⁶⁾The amount of a fresh liquid supplied.⁽¹⁷⁾Air permeability (sec/100 cc) converted to that of a 20 μm-thickmembrane.

As shown in Table 3, the microporous thermoplastic resin membranes ofExamples 6-11 produced by the method of the present invention wereparticularly excellent in appearance and also excellent in porosity, airpermeability and thermal shrinkage resistance. On the other hand, themicroporous membranes of Comparative Examples 6-8 had poor appearancebecause of striped suction spots on the surface, because the contacttime of the washed molding with the suction roll exceeded a rangerepresented by the above general formula (1). Further, ComparativeExample 6 was poorer in porosity and air permeability than Example 7with the same conditions of removing the membrane-forming solvent, andComparative Example 7 was also poorer in porosity and air permeabilitythan Example 6 with the same conditions of removing the membrane-formingsolvent.

EFFECT OF THE INVENTION

The first and second production methods of the present invention canquickly provide microporous thermoplastic resin membranes, whilesuppressing the evaporation of washing solvents used for removingmembrane-forming solvents and the shrinkage of the membranes. Themicroporous thermoplastic resin membranes obtained by the first andsecond production methods of the present invention have not onlyexcellent porosity, air permeability and thermal shrinkage resistance,but also excellent appearance because of no water spots (blister-likespots) and suction spots. The microporous membranes having suchproperties are suitable for battery separators, filters, etc.

1. A method for producing a microporous thermoplastic resin membranecomprising the steps of extruding a solution obtained by melt-blending athermoplastic resin and a membrane-forming solvent through a die,cooling an extrudate to form a gel-like molding, removing saidmembrane-forming solvent from said gel-like molding by a washingsolvent, and removing said washing solvent, wherein said washing solventhas (a) a surface tension of 24 mN/m or less at a temperature of 25° C.,(b) a boiling point of 100° C. or lower at the atmospheric pressure, and(c) a solubility of 600 ppm (on a mass basis) or less in water at atemperature of 16° C.; and wherein said washing solvent remaining in thewashed molding is removed by using warm water.
 2. A method for producinga microporous thermoplastic resin membrane comprising the steps ofextruding a solution obtained by melt-blending a thermoplastic resin anda membrane-forming solvent through a die, cooling an extrudate to form agel-like molding, removing said membrane-forming solvent from saidgel-like molding by a washing solvent, wherein a poor solvent to saidwashing solvent is caused to pass through the washed molding by asuction means to remove said washing solvent in a state that the washedmolding is in contact with said poor solvent; and wherein the contacttime t (seconds) of said washed molding with said suction means is in arange meeting the following general formula (1):t≦(100−T)³/(1,100×P ^(0.5)×logL)  (1), wherein T is the temperature (°C.) of said poor solvent, P is a suction pressure (kPa), and L is thesize of penetrating apertures of said suction means for sucking saidwashing solvent [diameter (μm) of the largest circle inscribed in saidpenetrating apertures].